Object lens and optical head for reproducing data from optical disks in different thickness of substrate

Abstract

The objective lens has a numerical aperture NA=0.6 when reproducing a DVD having a thickness of 0.6 mm and NA=0.42 when reproducing a CD having a thickness of 1.2 mm. At a boundary of NA 0.42, a slight stepped portion is formed so as to establish a phase difference. In this case, a designed wavelength is 0.635 .mu.m, and an optimum designed substrate thickness of a central portion is approximately 0.8 mm. While the DVD is reproduced, wave front aberration is on the order of 0.025.lambda., and jitter contained in the reproduction signal of the CD is equivalent to the present jitter. As a consequence, such an objective lens and an optical head with employment of this objective lens are provided, which precisely reproduce signals from the CD having the substrate thickness of 1.2 mm, and the DVD having the substrate thickness of 0.6 mm.

Claims

1. An optical head comprising: a light source; an objective lens for focusing light from said light source to an optical information recording medium; a light branching element for branching reflection light reflected from an optical recording medium from a light path returned to the light source; a photodetector; and means for positioning a focusing spot onto an information pit array of the optical recording medium; wherein:

a root mean square wave front aberration W.sub.rms, a wavelength.lambda..sub.2 of the light source, and a numerical aperture NA.sub.2 of the objective lens satisfy; ##EQU12## where "n.sub.1 " and "n.sub.2 " are refractive indices of an information recording medium substrate with respect to wavelengths.lambda..sub.1 and.lambda..sub.2; "d" denotes a depth of an information pit; a light source wavelength of an optical head for recording or reproducing information from said optical information recording medium is under such a degree that aberration can be optically neglected; and the root mean square wave front aberration can be optically neglected for a numerical aperture NA.sub.1.

2. An objective lens for focusing laser light onto an information recording film surface in order to record or reproduce information on or from an optical information recording medium for optically recording/reproducing the information, wherein:

a thickness of said objective lens for focusing said laser light independently having aberration of the same conditions is different at a central portion and a peripheral portion thereof.

3. An objective lens as claimed in claim 2, focusing laser light having a wavelength of.lambda..sub.2 on the information recording film surface in order to record or reproduce the information from two sorts of optical information recording mediums having different substrate thicknesses, different information pit hole depths, and different recording densities; wherein:

a peripheral portion of the objective lens generates no aberration when the laser light is focused through a substrate thickness of a first optical information recording medium having a large recording density; a central portion of the objective lens generates no aberration when the laser light is focused through a substrate thickness of a second optical information recording medium having a low recording density; a focused spot root mean square wave front aberration of incident light in combination with light passing through the central portion is lower than or equal to approximately 0.04 times a wavelength of laser light under use with respect to the substrate of the first optical information recording medium; and while recording or reproducing the second optical information recording medium by way of light passing only through the central portion of the objective lens, a numerical aperture NA.sub.2 only about the central portion, a substrate thickness by which the light of said central portion is collected without any aberration, and also wave front aberration W.sub.rms caused by an error in the substrate thickness of the second optical information recording medium substantially satisfies the following formula in comparison with an optical head having a light source wavelength.lambda..sub.1 under which information is optically recorded on or reproduced from the second optical information recording medium without any aberration under better conditions, using a normal objective lens of a numerical aperture NA.sub.1: ##EQU13## where "n.sub.1 " and "n.sub.2 " denote refraction indices of an information recording substrate with respect to the wavelengths.lambda..sub.1 and.lambda..sub.2, respectively.

4. An objective lens as claimed in claim 3 wherein

the substrate thickness of the second optical information recording medium is approximately 1.2 mm, and substantially satisfies the following expression: ##EQU14##

5. An objective lens as claimed in claim 2 wherein

said objective lens substantially satisfies the Abbe's sine condition as an overall objective lens.

6. An objective lens as claimed in claim 2 wherein

a shape of a boundary portion between the peripheral portion and the central portion is smoothly connected.

7. An objective lens as claimed in claim 2 wherein

an optimum substrate thickness of the central portion is continuously variable in a coaxial shape from a lens center.

8. An objective lens as claimed in claim 2 wherein

a thin film is loaded on either the peripheral portion or the central portion.

9. An objective lens as claimed in claim 2 wherein

said objective lens is arranged by a lens whose substrate thickness is optimized over an entire surface, and a parallel plate arranged on the object side of the lens, a hole being formed in a central portion of said parallel plate.

10. An objective lens as claimed in claim 2 wherein

a paraxial focus of the objective lens on the side of the light source is separated from a surface of the objective lens on the side of the light source by greater than or equal to 2 mm.

11. An optical head comprising: a light source; the objective lens recited in claim 10, for focusing light from said light source to an optical recording medium; a light branching element for branching reflection light reflected from an optical recording medium from a light path returned to the light source; a photodetector; and means for positioning a focused light spot onto an information pit array of the optical recording medium; wherein:

in order to reproduce information recorded on a substrate having a thickness of 1.2 mm, the optical head further comprises means for reducing a light amount of light or for interrupting the light entered into a peripheral portion of the objective lens; and a galvanometer mirror employed as an actuator for tracking the focused light spot to the information pit array on the optical recording medium; wherein a rotary shaft of the galvanometer mirror is arranged near the paraxial focus of said objective lens on the side of the laser light source.

12. An optical head for recording or reproducing information by employing an objective lens for focusing laser light onto an information recording film surface in order that information is recorded on or reproduced from two sorts of optical information recording mediums having different substrate thicknesses, different information pit hole depths, and different recording densities, wherein:

a thickness of said objective lens independently having aberration of the same conditions is different at a central portion and a peripheral portion thereof.

13. An optical head as claimed in claim 12 wherein

in the case that the information recorded on a second optical information recording medium having a low information recording density is reproduced, a light amount of light entered into a peripheral portion of said objective lens is reduced or interrupted, whereas in the case that a first optical information recording medium having a high information recording density is reproduced, a reduction or an interruption of an incident light amount is not carried out.

14. An optical head as claimed in claim 12, wherein

means for reducing the light amount of the light or interrupting the light entered into the peripheral portion of said objective lens is made with the objective lens in an integral form.

15. An optical head as claimed in claim 13, wherein

in the case that the second optical information recording medium having a low information recording density is recorded or reproduced, in order to trace the collective spot onto the information recording pit array of the information recording film surface, means for reducing a light amount of light or interrupting the light entered into a peripheral portion of said objective lens is simultaneously moved in conjunction with movement of said objective lens.

16. An optical head as claimed in claim 10, wherein

a substrate thickness of said first information recording medium having the high information recording density is 0.6 mm, and a substrate thickness of said second information recording medium is 1.2 mm.

17. An optical head as claimed in claim 12, wherein

in the case that the information recorded on said second optical information recording medium having the low information density is reproduced by employing the objective lens recited in any of claim 2 to claim 9, a dimension of the photodetector and a dimension of the optical system are set in such a manner that the light entered into the peripheral portion of said objective lens is sufficiently reduced on the photodetector.

18. An optical head comprising: a light source; an objective lens for focusing light from said light source to an optical recording medium; a light branching element for branching reflection light reflected from the optical recording medium from a light path returned to the light source; a photodetector; and means for positioning a focused light spot onto an information pit array of the optical recording medium; wherein:

in order to optically record/reproduce information on/from the optical recording medium through a transparent parallel plate substrate, a paraxial focus of the objective lens for focusing laser light onto an information recording film surface on the side of the light source is separated from a surface of the objective lens on the side of the light source by greater than or equal to 2 mm; said optical head further comprising means for reducing a light amount of light or for interrupting the light entered into a peripheral portion of the objective lens; and a galvanometer mirror employed as an actuator for tracking the focused light spot to the information pit array on the optical recording medium; wherein a rotary shaft of the galvanometer mirror is arranged near the paraxial focus of said objective lens on the side of the laser light source.

19. An optical head comprising: a light source; an objective lens for focusing light from said light source to an optical recording medium as a reproducing spot thereon; a light branching element for branching reflection light reflected from the optical recording medium from a light path returned to the light source; a photodetector; and means for positioning a collective spot onto an information pit array of the optical recording medium; wherein

signals are reproduceable from two sorts of optical information recording mediums having different substrate thicknesses, different information pit hole depths, and different recording densities without replacing the objective lens; and when a first optical information recording medium having a high information recording density is reproduced, a ratio of a total light amount of the reproducing spot on said first optical information recording medium to a total incident light amount of a lens portion is greater than or equal to 90%.